This invention relates to semiconductor devices and, more particularly, to buried heterostructure semiconductor light emitting devices such as lasers and LEDs.
A common problem in the fabrication of low threshold buried heterostructure (BH) lasers in the InGaAsP/InP materials system is the control of leakage currents (i.e., currents which bypass the active region of the device). These currents lead to high lasing threshold, low differential quantum efficiency, abnormal temperature dependence of threshold current, and rollover of the light-current (L-I) characteristic. All of these factors have a significant negative impact on the use of BH lasers in transmitters for fiber optic communication systems.
One possible solution to the problem of leakage current in buried heterostructure lasers is the controlled introduction of high resistivity material into the device structure. This high resistivity material could be used to block current flow through undesired leakage paths. Previously, high resistivity liquid phase epitaxial (LPE) Al.sub.0.65 Ga.sub.0.35 As (lightly Ge-doped) material has been utilized for current confinement in AlGaAs/GaAs buried heterostructure lasers, but subsequent attempts to produce high resistivity LPE InP material for this purpose have not been successful. Deuteron bombardment has also been shown to produce highly resistive material from p-type InP, but this material is not expected to remain highly resistive during subsequent processing. In particular, because the high resistivity is related to deuteron implant damage, the resistivity anneals out at the high temperatures (e.g, above about 600.degree. C.) required for subsequent LPE growth.
In addition, bifurcated, reverse-biased p-n junctions have also been reported for constraining current to flow through the active region of InGaAsP/InP lasers. These blocking junctions have been fabricated by the implantation of Be with n-InP substrates, by the diffusion of Cd into n-InP substrates, and by the epitaxial growth of a p-InP layer onto an n-InP substrate. But, all of these devices are impaired to some extent by leakage currents because of the imperfect blocking characteristics of the reverse-biased junctions.
More recently, D. P. Wilt et al. reported in Applied Physics Letters, Vol. 44, No. 3, p. 290 (Feb. 1984) that InP/InGaAsP CSBH lasers with relatively low leakage currents and low lasing thresholds can be fabricated by incorporating into the structure a high resistivity Fe-ion-implanted layer which constrains pumping current to flow through the active region. The high resistivity layer is produced by an Fe-ion implant into an n-type InP substrate followed by an annealing treatment prior to LPE growth. This laser is also the subject of copending application Ser. No. 549,160 filed on Nov. 8, 1983 by R. J. Nelson et al. Although the resistivity of the Fe-ion-implanted layer is stable even after being subjected to the high temperatures characteristic of LPE growth, the thinness of the Fe-implanted layer (about 0.4 .mu.m) renders it difficult to reproducibly position the thin active layer (about 0.1-0.2 .mu.m thick) adjacent thereto. When the active layer is not so placed, shunt paths are created which allow leakage current to flow around the active layer. Hence, high performance (low threshold, high efficiency) devices are hard to fabricate reproducibly.